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summary: replaced 2 carrier seer 12 with 2 lennox seer 17 & 18.6 with 2 spd compressor & var. spd air handler. ducts, which never sweat before, now are dripping from the bottom of the supply trunk in the attic... and the sheetrock under the soffit in the garage containing the supply trunk for the first floor was dripping. trunks are round metal wrapped with fiberglass, taped and mastic.

Contractor says reinsulate ducts. Forgot to mention, old air handlers were 4 ton, new are 3 ton. I shut off humidity control and set fan delay to 100% on then 60 second shutdown.

Now garage ceiling has dried except for one spot and attic dripping SEEMS to have lessened.

upstairs lennox model HPXA19-036
fan setting=1400, SEER 17

downstairs lennox model HPXA19-038
fan setting=1275, SEER 18.6

Air handlers are both CBX32MV-036

questions:
what is proper cfm per ton (heard range of 400-450)
what is effect of increasing cfm?
what is upper limit you should set cfm for cooling or heating?
docs are not clear which has preference: stage one cooling fan speed at 70%, or delay setting of 100%....anyone know?
finally: what is proper temp diff from return to supply?

Does Lennox have a problem with my equipment they are not disclosing? Factory refers back to contractor. Contractor has set up visit from factory rep next week, but has not responded to request for conference call with factory engineers.

I am located at the beach in SC, high temp and humid, but ducts never had condensation with carriers.

If it were air leaks, it seems to me the ducts would have dripped with the Carriers as well.... also, dripping is the length of the supply ducts, both in the attic and the garage ceiling. It would seem something else must be different... duct air temp maybe?

Old air handlers were 4 ton, new are 3 ton and cfm's may be different... that's why I am trying to find out "what is the correct cfm per ton?" The lennox are easily variable and have an indicator to count cfms. Thanks for your comment.

Did you change out the duct system with the new equipment. If not, you may need to insulate the duct better. This will help the outside of the ducts from getting too cold and producing condensation when in contact with the warmer air in the attic and crawl spaces. In Wilmington, NC we usually see condensation on ducts in warm, humid crawl spaces. We have also see condensation problems when contractors have put varialbe speed equipment on old, poor insulated duct systems.

I'm not sure about the current Lennox systems, but the humidity controls on the older systems would call for second stage cooling & slowest fan speed possible, which would produce very cold (<45F) air, which will be below the dewpoint of your area. I'm assuming the old Carrier system was a single stage system, so you were probably getting supply temps at least 10-20F warmer than that. Could be leaks, could be not enough insulation.

Thanks, yellow dot. Can you tell me at what point there are ill effects from increasing cfm's? My theory is if the cfm's increase; the temp in the duct will decrease (perhaps stopping the condensation) but I surmise at the expense of humidity control. Is there a cfm per ton you wouldn't want to exceed?

System currently maintains 74 degree setpoint with indside humidity (per humidistat on thermostat) at between 58 and 62 percent. Contractor recommended "precision" setting at 50%, but that's when the sweating began. Humidity control now off.

Thanks, beachhvac.. ducts in attic were new in 1998. ducts in garage ceiling new last year. Is there life expectancy for fiberglass duct wrap?

Sides and top of duct insulation in attic can be depressed. Bottom of attic duct seems not to have any "give" and insulation may be compressed. I notice plastic hanger straps are 1/2" wide and I have read that 1 1/2" should be considered a minimum.

I can get to the attic ducts relatively easily. Would you recommend cutting off the entire wrap or just the wet portion before re-insulating? I am told two vapor barriers are not a good idea.

The garage ceiling would have to be opened up to insulate the first floor supply duct.... very expensive... especially since they were new last year.

Really hard to say which is worse as the first floor ducts are concealed. I cut inspection hole in side of soffit directly under air handler. replaced wet insulation under duct. sweating sheetrock under garage supply trunk seems to have stopped, except at a nail or two, which no doubt is absorbing heat and allowing condensation.

We have a commercial dehumidifier running in the attic and sweating seems to have diminished there, although impossible to tell if from increased cfm's or dehumidifier.